iwlwifi: mvm: add the skb length to a print

When printing a TX, add to the print the length of the frame.
That will help with BSEP (buffer status report poll) tests.

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>